1. Field of the Invention
This invention relates to a process for purifying crude p-aminophenol which is particularly suitable for removing 4,4'-diaminodiphenyl ether from the crude p-aminophenol. In a specific aspect, the present invention is concerned with a process for purifying crude p-aminophenol obtained by catalytic reduction of nitrobenzene in an aqueous sulfuric acid medium. The pure p-aminophenol obtained according to the process of the invention is useful as a starting material for medicines.
2. The Prior Art
As well known, p-aminophenol has been used from old as an intermediate for an antioxidant of dyes or a photographic developer. In recent years, since aminopyrine or phenacetin (i.e., acetophenetidine) generally used as an antipyretic and anodyne is encountered with a problem of side effect, there has been a rapidly growing demand for harmless p-N-acetylaminophenol. Thus, p-aminophenol becomes important as an intermediate for p-N-acetylaminophenol.
Para-aminophenol has been heretofore produced from p-nitrobenzene. In recent years, a process of catalytically reducing nitrobenzene in an aqueous sulfuric acid medium has been newly developed and has been reduced into practice on an industrial scale. Para-aminophenol obtained by the catalytic reduction of nitrobenzene generally contains aniline, 4,4'-diaminodiphenyl ether (hereinlater referred to simply as DADPE) and other by-products as described in French Pat. No. 1,564,882. Removal of these impurities will be necessary depending on the purpose for which the aminophenol is intended to apply. However, the removal of such impurities are difficult.
For instance, French Pat. Nos. 1,564,882 and 2,029,142 describe that the crude p-aminophenol obtained by the catalytic reduction of nitrobenzene in aqueous sulfuric acid medium is contacted with aliphatic, alicyclic or aromatic ketones including acetone to remove the impurities from the p-aminophenol by extraction. In British Pat. No. 1,028,078, there is described a method in which the crude p-aminophenol is washed with an aliphatic alcohol to remove the impurities therefrom. However, the solvents of the type indicated above dissolve p-aminophenol as well as the impurities, so that complete removal of the impurities results in a relatively great loss of p-aminophenol.
Japanese Patent Laid-open No. 46-411 describes an improved process which comprises contacting p-aminophenol in aqueous acid medium in a specific pH range of 4.8-5.8 with an aprotic solvent such as methylene dichloride which is immiscible with water and can selectively dissolve the impurities such as DADPE, thereby removing the impurities by extraction. However, this process is also disadvantageous in that the solubility of the impurities, particularly DADPE, in such solvent is very small as stated in the specification, so that complete removal of the impurities requires large amount of the solvent and repetitions of extraction. Even though there is used such solvent as to selectively dissolve DADPE and other impurities, use of such solvent in large amount will result in a loss of p-aminophenol in appreciable amount. In addition, the repetitions of extraction is inconvenient in an industrial sense.